About 75% of cold-formed seamless pipes, and almost 100% of pipes, rods and wires of a diameter less than 4 mm are produced throughout the world by drawing, i.e. by a relatively simple process in which a metal billet is pulled through a drawing die. The main disadvantage of this process is that it has to be carried out in several stages or cycles, notably in the production of small-diameter pipes and rods where the required sizing of the final product is achieved only after a considerable number of stages. The need for a multi-stage operation stems from the known fact that in a single drawing stage, the deformation expressed by the ratio of a cross-section area of an elongated body before and after drawing cannot exceed about 55%. Accordingly, for the production of, for example, seamless capillary steel pipes, as many as about 10 to about 20 drawing stages may be required. Such a multi-stage process requires a variety of intermittent auxiliary operations such as thermal treatment, etching, straightening, cutting, etc. all of which impair the degree of precision with which the final body is formed as well as the surface quality thereof. In addition, the multi-stage nature of the process in combination with the auxiliary operations augments the volume of the required industrial facilities and leads to increased consumption of metal, energy and working time. Also, at least some of the auxiliary operations are ecologically hazardous. Finally, the drawing process for the formation of elongated bodies is inadequate for the manufacture of a variety of specific products such as, for example, pipes and rods from brittle or super-hard metals, thick walled pipes with a relatively small and precise inner channel, high precision pipes and others.
In the production of metal pipes and rods, the above shortcomings may be at least partly overcome by employing so-called hydrostatic extrusion, also known as hydroextrusion. Hydroextrusion is a process in which a billet is squeezed through a die mounted in a wall of a pressure chamber filled with a pressurized working liquid serving for the creation of a working pressure that exceeds the yield point of the processed metal. As known to those skilled in the art, by this technique, it is possible to achieve in one single stage a deformation that exceeds 55%. However, this process in which the driving force for the extrusion is provided exclusively by the pressurized working liquid, is slow, non-stable and expensive and accordingly not widely used.
In an effort to improve the productivity of hydroextrusion, it has been proposed to combine the drawing and hydroextrusion techniques into one processing cycle for the continuous production of wire, rods and pipes.
U.S. Pat. No. 3,747,384 describes a method of reducing the cross-sectional area of a body by means of hydroextrusion with the simultaneous application of a pulling force. In accordance with that method extrusion, which proceeds via a conical extrusion die, proceeds by the action of two driving forces, to wit hydrostatic pressure and pulling force, which act in combination on the billet passing through one and the same die. The deformation of the billet achieved in this way does, however, not correspond to the maximal sum of deformations which might be provided separately by hydroextrusion at the given pressure and drawing at the given pulling force, because, for the optimization of these two forces, different conical shapes of the die are required.
U.S. Pat. No. 3,841,129 describes a continuous hydrostatic extrusion process and apparatus wherein in a first phase a stock wire is drawn through a drawing die into a high pressure chamber with a hydroextrusion die and is spooled therein, and in a second phase the partly formed wire which remains under hydrostatic pressure is paid off and drawn through the hydroextrusion die. A single apparatus unit thus performs a first forming operation by drawing and a second, subsequent forming operation by hydrostatic extrusion, and by using a series of two or more such units, the forming operation may be performed in several stages. Basically, the second forming operation of U.S. Pat. No. 3,841,129 is equivalent to the process of U.S. Pat. No. 3,747,384.
SU 1,726,083 A1 describes a method and equipment for the continuous forming of elongated tubular billets into pipes applying in combination hydroextrusion and drawing. The equipment includes a high pressure chamber with inlet port for a tubular billet and with a hydroextrusion die, and pulling means for applying pulling force to the forward end of the billet protruding from the hydroextrusion die, and basically this method is similar to that of U.S. Pat. No. 3,747,384.
The above specified methods and equipment are typical of the prior art and as mentioned have an intrinsic shortcoming that in one operational cycle the deformation does not yield the maximal possible deformation at the given energy consumption. In addition, these methods are inadequate for the production of thin-walled pipes since the elevated liquid pressure inside a pipe would tear it at the exit from the hydroextrusion die where the pressure around the pipe drops down to atmospheric. Still further, in the prior art equipment, the operations cannot be adjusted to the requirements of specific materials such as hard and brittle metals and to other needs of a specific case. Still further, the known combined methods are inadequate for the production of pipes and other elongated products with a high degree of precision and good surface quality.
A further problem encountered in hydroextrusion is the so-called shot effect which occurs when the rear end of a pressure formed elongated body is forced away from the hydroextrusion die by the action of the pressurized liquid inside the pressure chamber. It has been proposed to avoid the shot effect by providing the billet with a flaring rear end portion which is retained by the hydroextrusion die at the end of the extrusion operation, i.e. when the billet would otherwise be ejected from the high pressure chamber. By another known method for avoiding the shot effect, the pressure within the pressure chamber is abruptly reduced when the rear end of the billet reaches the hydroextrusion die. By either of these methods the shot effect may be eliminated but they have the common drawback that at the end of a cycle, the hydroextrusion die must be dismantled for removing therefrom the stuck end of the billet, which is an obvious complication that affects the efficiency of the operation and leads to a loss of metal.